1. Field of the Invention
The present invention relates to a glycolic acid copolymer. More particularly, the present invention is concerned with a glycolic acid copolymer comprising (a) glycolic acid monomer units as a main component, (b) non-glycolic, hydroxycarboxylic acid monomer units, and optionally (c) diglycolic acid monomer units in not more than a specific amount, the non-glycolic, hydroxycarboxylic acid monomer units (b) constituting a plurality of segments each independently consisting of at least one non-glycolic, hydroxycarboxylic acid monomer unit (b), wherein the segments have an average chain length of from 1.00 to 1.50 in terms of the average number of non-glycolic, hydroxycarboxylic acid monomer unit or units (b), wherein the glycolic acid copolymer has a weight average molecular weight of 50,000 or more. The glycolic acid copolymer of the present invention is a high quality, high molecular weight product which is advantageous not only in that the copolymer enables production of a shaped article exhibiting excellent gas barrier property, satisfactory mechanical strength and satisfactory biodegradability, but also in that the copolymer exhibits high heat stability, thereby greatly suppressing the occurrence of discoloration even when melt-shaped at high temperatures. The present invention is also concerned with a method for producing the above-mentioned glycolic acid copolymer efficiently and stably.
2. Prior Art
In recent years, the problem of plastic materials wastes have been attracting attention from the viewpoint of environmental protection. For facilitating environmental protection, there is an increasing demand for polymers which can be spontaneously degraded in natural environment and for shaped articles produced from such polymers. A polyglycolic acid and a glycolic acid copolymer have not only a good balance of heat resistance, mechanical strength and biodegradability, but also extremely excellent, gas barrier property. By virtue of such excellent properties, polyglycolic acids and glycolic acid copolymers are attracting attention as biodegradable polymeric materials which are suitable for producing packaging materials, such as containers and films. Therefore, in the art, studies have been being conducted for developing a high molecular weight polyglycolic acid and a high molecular weight glycolic acid copolymer which have a satisfactory mechanical strength required of shaped articles.
However, a polyglycolic acid and a glycolic acid copolymer having a high glycolic acid monomer unit content have high melting temperatures and, hence, these polymers have defects not only in that a high temperature is necessary for melt shaping these polymers, but also in that the difference between the melting temperature and the decomposition temperature is small. Therefore, problems due to poor heat stability have been encountered in that these polymers exhibit marked discoloration at the time of melt shaping, that these polymers have poor resistance to heat aging, and that these polymers generate heat decomposition products when heated.
In an attempt to solve such problems, methods using a phosphorus compound as a discoloration inhibitor have been proposed. However, these methods have not enabled production of a polyglycolic acid and a glycolic acid copolymer which have satisfactory heat stability.
For improving the heat stability of a polyglycolic acid and a glycolic acid copolymer (hereinafter, these polymers are frequently, collectively referred to as “glycolic acid polymer”), a method has been proposed in which the terminal functional groups of a glycolic acid polymer are reacted with a specific compound (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 56-157422). This method is effective for suppressing the depolymerization of a glycolic acid polymer at the time of shaping. However, this method does not exhibit a satisfactory effect of suppressing the discoloration at the time of shaping.
For solving the problem of the poor heat stability of a polymer, there is a generally employed measure in which copolymerization is performed for producing a copolymer having a lowered melting temperature. This measure has also been employed with respect to a glycolic acid polymer, and studies have been performed for producing various glycolic acid copolymers having a lowered melting temperature.
For example, there has been proposed a method for producing a glycolic acid copolymer having a high molecular weight, in which many steps are performed, as follows: glycolic acid and/or a derivative thereof is subjected to dehydration condensation to obtain a dehydration condensation product; the obtained dehydration condensation product is subjected to thermal decomposition to produce a cyclic dimeric glycolate (so-called “glycolide”); the obtained glycolide is purified to a high degree; and the resultant highly purified glycolide is subjected to ring-opening polymerization with, e.g., cyclic dimeric lactate (so-called “lactide”) in the presence of a catalyst, thereby obtaining a glycolic acid copolymer (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 48-62899 (corresponding to U.S. Pat. No. 3,839,297 and GB1416196 A)).
However, a glycolic acid copolymer (such as a glycolide-lactide copolymer) obtained by the above-mentioned method using a ring-opening polymerization is likely to have a structure in which comonomer units (e.g., lactic acid monomer units in the above-mentioned glycolide-lactide copolymer) are introduced as polymer blocks into the primary structure of the polymer. Therefore, when the amount of the comonomer units is small, a satisfactory effect of lowering the melting temperature of a polymer cannot be obtained and, hence, the effect of suppressing the discoloration at the time of shaping becomes unsatisfactory.
On the other hand, when the amount of comonomer units (such as lactic acid monomer units) is increased for lowering the melting temperature of a glycolic acid copolymer, the gas barrier property, which is characteristic of a glycolic acid copolymer, tends to become poor.
As another method for producing a glycolic acid copolymer by using a ring-opening polymerization, a method has been proposed in which glycolide is copolymerized with, e.g., ε-caprolactone, trimethylene carbonate, p-dioxanone or a copolymer compound, such as a cyclic dimeric ester of glycolic acid with a malate (see Unexamined Japanese Patent Application Laid-Open Specification No. 3-269013 (corresponding to DE3335588 A, GB2127839 A and U.S. Pat. No. 4,605,730), Examined Japanese Patent Application Publication No. 63-47731 (corresponding to GB2033411 A, DE2850824 A and U.S. Pat. No. 4,243,775), Unexamined Japanese Patent Application Laid-Open Specification No. 9-12689 (corresponding to EP751165 A2 and U.S. Pat. No. 5,633,343) and Unexamined Japanese Patent Application Laid-Open Specification No. 2-209918).
On the other hand, as another method for producing a glycolic acid copolymer, there is known a method in which, for example, mainly glycolic acid and/or a derivative thereof is polycondensed with a comonomer. The method using a polycondensation is commercially more advantageous than the method using a ring-opening polymerization, in that the former involves fewer steps than the latter. Further, the method using a polycondensation is advantageous in that comonomer units can be randomly introduced into the primary structure of the resin. Therefore, the method using a polycondensation is highly effective for improving the properties of a glycolic acid copolymer. For example, by the method using a polycondensation, the melting temperature can be greatly lowered by introducing a small amount of comonomer units. For the reason of these advantages, the method using a polycondensation has been considered as a promising polymerization method capable of producing a glycolic acid copolymer having satisfactory properties with respect to shapability and gas barrier property, and various studies have heretofore been made on the method using a polycondensation. Various glycolic acid copolymers produced by this method have been proposed, as follows.
For example, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. 7-501102 (corresponding to WO93/10169) discloses a copolymer obtained from glycolic acid and a polycarboxylic acid containing 2 or more carboxyl groups per molecule. However, this patent document has no description about a high molecular weight copolymer (for example, a copolymer having a weight average molecular weight of 50,000 or more). In Unexamined Japanese Patent Application Laid-Open Specification No. 11-255873, a copolymer is proposed which is comprised of hydroxycarboxylic acid monomer units, aliphatic dicarboxylic acid monomer units and ethylene oxide/propylene oxide block copolymer units and which has a weight average molecular weight of from 50,000 to 1,000,000. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 8-3296 proposes an aliphatic polyester copolymer which is comprised of aliphatic hydroxycarboxylic acid monomer units, aliphatic diol monomer units and aliphatic dicarboxylic acid monomer units and which has a number average molecular weight of from 10,000 to 100,000. In these patent documents, copolymers containing lactic acid monomer units as hydroxycarboxylic acid monomer units are exemplified, and working examples are described in which such a copolymer is shaped into a fiber, sheet or the like. However, the copolymers exemplified in these patent documents have a problem in that, when a copolymer contains a large amount of glycolic acid monomer units, the heat stability of the copolymer is not satisfactory and, hence, a marked discoloration is likely to occur at the time of melt shaping.
There has also been proposed an aliphatic polyester which is comprised of glycol monomer units and at least one member selected from the group consisting of glycolic acid monomer units and lactic acid monomer units and which has a solution viscosity of 0.35 or more (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 1-156319). However, such copolymer has low heat stability and exhibits marked discoloration at the time of melt shaping.
Further, there has been proposed a copolymer which is obtained by a transesterification reaction between (A) a polyglycolic acid or a copolymer of glycolide and lactide and (B) a polyester comprised of diglycolic acid monomer units and diol compound monomer units, wherein polyester (B) is used in an amount of from 2 to 50% by weight, based on the total weight of raw material (A) and polyester (B) (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 52-147691 (corresponding to GB1572362, U.S. Pat. No. 4,048,256, U.S. Pat. No. 4,095,600, U.S. Pat. No. 4,118,470 and U.S. Pat. No. 4,122,129)). However, the heat stability of the copolymer is not satisfactory, and a marked discoloration is likely to occur especially at the time of melt shaping.
Unexamined Japanese Patent Application Laid-Open Specification No. 9-808220 (corresponding to WO97/08220) discloses a method for producing a polyhydroxycarboxylic acid which has a weight average molecular weight of 50,000 or more, in which a hydroxycarboxylic acid or an oligomer thereof is subjected to polycondensation in the presence of an inorganic solid acid catalyst and an alkaline earth metallic compound catalyst. However, the polyhydroxycarboxylic acid obtained by this method is discolored to assume a pale brown color and, hence, has a poor quality. In addition, the heat stability of the polyhydroxycarboxylic acid is low, and the discoloration at the time of melt shaping is marked.
Unexamined Japanese Patent Application Laid-Open Specification No. 11-130847 (corresponding to WO99/19378) discloses a method for producing a high molecular weight polyglycolic acid, in which a hydrolyzate of methyl glycolate is subjected to polycondensation to obtain a prepolymer and, then, the obtained prepolymer is subjected to solid phase polymerization. However, the obtained polyglycolic acid exhibits poor heat stability and marked discoloration at the time of melt shaping.
As described hereinabove, there has been totally unknown a glycolic acid copolymer containing a large amount of glycolic acid monomer units, which is advantageous not only in that it exhibits high heat stability at the time of melt shaping, but also in that it enables production of a shaped article exhibiting high mechanical strength and excellent gas barrier property.